No, buckskins can have a dorsal stripe. It's just called countershading.
And a horse can be a "dunskin", so buckskin with dun markings, which was probably the case of your mare, jyuukai, or she was just a buckskin with countershading.
Abby is shedding out a decent amount finally, and she has random sootiness on her. Particularly, her jaw, withers, shoulders, and hips. It looks like she rolled around in dirt, but it's dark hairs. I imagine she'll continue to shed out and then lighten up to normal.